What does an Infectious Diseases specialist do, and when should you see one?

In short

ID specialists treat serious, unusual or hard-to-treat infections.
These include TB, HIV, PUO, and bone, fungal, parasitic, surgical site and transplant-related infections, and infections in people with weak immunity.
Lab results need expert reading: positive isn’t always infection, negative isn’t always clear.
Medicines are chosen to suit you and your other medicines.
They also advise on adult vaccines, such as flu, pneumococcal, hepatitis B and shingles.
They also guide antibiotic use and infection control in hospitals.
Confused, breathless or faint with an infection? Call 112.

Most infections, like a cold, a simple urine infection or a fever that settles in a few days, are treated well by your family doctor. But some infections are serious, unusual, hard to diagnose, or simply refuse to get better. That is where an Infectious Diseases specialist (ID specialist) comes in.

An ID specialist is a doctor who focuses only on infections: illnesses caused by bacteria, viruses, fungi and parasites. Think of them as detectives for infections.

Who is an Infectious Diseases specialist?

After MBBS, these doctors do several more years of training focused on infections. In India, the main qualifications are:

DM in Infectious Diseases. Usually a three-year course after an MD in General Medicine or Paediatrics (a few institutions also accept related degrees, such as Tropical Medicine).¹ Some institutions also offer longer courses that start directly after MBBS.
DrNB in Infectious Diseases, from the National Board of Examinations in Medical Sciences.

Some hospitals and universities also run fellowships in Infectious Diseases. These vary a lot in length and format.

The specialty covers all kinds of germs, including bacteria, viruses such as HIV and hepatitis, fungi and parasites, in every part of the body.

What does an ID specialist do?

Finds the cause. For a long fever with no clear reason, or an infection that keeps coming back, they piece together your history, examination and test results like a detective.
Picks the right medicine. Not just which antibiotic, antiviral or antifungal, but also the dose, whether by mouth or drip, and for how long.
Treats infections that resist common antibiotics.
Looks after long-term infections such as HIV, tuberculosis (TB), and hepatitis B and C.
Cares for people with weak immunity, such as transplant and cancer patients.
Prevents infections through vaccines for adults and travellers (more on this below), and preventive medicines after certain exposures.

Conditions an ID specialist commonly treats

ID specialists look after a wide range of infections, including:

Tuberculosis (TB), including drug-resistant TB and TB outside the lungs
HIV
Bone and joint infections, including infections around implants such as joint replacements
Fungal infections, such as mucormycosis (“black fungus”) and aspergillosis
Parasitic infections, such as malaria and worm infections
Surgical site infections, which develop in or around a wound after an operation
Pyrexia of unknown origin (PUO), a fever lasting weeks without a clear cause even after tests
Transplant-related infections, after a bone marrow or organ transplant
Infections in people with weak immunity, such as those on cancer treatment, long-term steroids or other immune-lowering medicines

They also treat hepatitis B and C, infections resistant to common antibiotics, and tropical fevers such as dengue, typhoid and scrub typhus.

Reading the lab report: positive doesn’t always mean disease, negative doesn’t always mean healthy

Many people read a lab report as a simple yes or no. In reality, no test is perfect. Any test can sometimes miss a disease that is there, or show a result when there is no disease. Doctors call these false negatives and false positives.² A big part of an ID specialist’s job is knowing how each test works, and reading the result alongside your symptoms and examination.

Why a negative test may not rule out an infection

The test was done too early or too late. For dengue, the NS1 test works best in the first few days of fever, while the antibody (IgM) test is more useful after about the fifth day. A negative test on the wrong day can be misleading.³
Antibiotics were taken before the sample. The germ may then not grow in the lab, even though the infection is still there.
The sample missed the germ. The germ may be in a part of the body the sample didn’t reach, or in numbers too small for the test to pick up. For example, a normal sputum test does not rule out TB that is affecting another part of the body.

Why a positive test may not mean an active infection

The germ is a contaminant. Skin bacteria can sneak into a blood sample while it is being taken.
The germ is living there harmlessly. Bacteria often live quietly in the urine, especially in older people or those with a urine catheter. Expert guidelines advise against treating this in most people who have no symptoms. The main exceptions are pregnancy and before certain urology procedures.⁴
The test shows a past infection or vaccine. The Widal test for typhoid looks for antibodies, which can stay positive after an old infection or a typhoid vaccine. A blood culture, which grows the germ itself, is more reliable.⁵
The test picks up leftovers. Very sensitive PCR tests detect a germ’s genetic material, not whether it is still alive. After COVID-19, for example, a PCR test can stay positive for weeks after recovery, usually because of leftover genetic material rather than live virus.^6,7

“Sensitive” next to an antibiotic is only part of the story, too. A medicine may kill the germ in the lab but not reach the infected place, such as the brain or bone, in high enough amounts. The specialist also checks whether that medicine is safe for you.

This is why doctors look at the whole picture: how likely the disease was before the test, your symptoms and examination, and what each test can and cannot show.² Reading results this way avoids two problems: missing a real infection, and giving antibiotics that are not needed.

Medicines that don’t mix well

People with serious infections often take other medicines too, for diabetes, heart disease, epilepsy, mental health, HIV, or to protect a transplant. Some infection medicines interact strongly with these. This can change which medicine an ID specialist chooses, or how the dose is adjusted. For example:

Rifampicin, a key TB medicine, speeds up how the body clears many other drugs. It can make some HIV medicines, blood thinners like warfarin, transplant medicines and birth control pills work less well.⁸
Some antifungal medicines do the opposite. Posaconazole, for example, raises the level of the transplant medicine tacrolimus so much that its dose usually has to be cut to about one-third, with regular blood tests.⁹

Kidney and liver function, pregnancy, age and past allergies also change which medicine is safe, and at what dose. This is why you should always tell your doctor about every medicine you take, including over-the-counter, Ayurvedic and herbal products.

Caring for people with weak immunity

Some people’s immune systems are weakened by an illness or by its treatment. They include:

people who have had a bone marrow (stem cell) or organ transplant
people on chemotherapy, especially for blood cancers
people living with HIV, especially if diagnosed late
people on long-term steroids or other immune-lowering medicines, including “biologic” drugs for conditions like rheumatoid arthritis or psoriasis
people with poorly controlled diabetes, or advanced kidney or liver disease
people without a working spleen, or born with immune problems

In these patients, infections can be caused by unusual germs, may show few signs (sometimes not even a fever), and can get worse within hours. India saw this during the second COVID-19 wave, when cases of mucormycosis (“black fungus”) shot up. In one study of 2,826 patients, 78% had diabetes and 87% had been given steroids.¹⁰

Prevention matters just as much. Before a transplant, chemotherapy or a biologic medicine, ID specialists often check for hidden infections like TB and hepatitis B that could flare up, update vaccines, and plan preventive medicines.

Vaccines are not just for children: adult immunisation

Vaccines protect adults too, not just children.

The Association of Physicians of India’s adult immunisation guidelines (2024) focus on influenza, pneumococcal, hepatitis B and shingles vaccines, especially for older adults, healthcare workers and people with long-term health conditions.¹¹ Depending on your age, health and lifestyle, these may include:

Influenza (flu), every year, because the virus keeps changing
Pneumococcal, which protects against a common cause of pneumonia and blood infections
Hepatitis B, if you are not already protected
Shingles (herpes zoster), for older adults
Tetanus and diphtheria boosters, including during pregnancy
HPV, which helps prevent cervical and some other cancers
Travel vaccines, such as typhoid or hepatitis A, depending on where you are going

An ID specialist can work out which vaccines you need, check whether you are already protected, and plan the timing. This matters most before a transplant, chemotherapy or a biologic medicine: vaccines work best when given beforehand, and some “live” vaccines are not safe once immunity is lowered.

Why this matters in India

Infections that resist antibiotics are a growing problem in India. A large study by the Indian Council of Medical Research (ICMR) looked at about 1.6 lakh patients in 20 big hospitals. Among patients with infections caused by four common hospital germs, about 6 in 10 had germs resistant to some of the strongest antibiotics available, called carbapenems. These patients were more likely to die in hospital: about 32 in 100, compared with about 22 in 100 when the germ could still be treated with these drugs.¹²

These were very sick patients in large hospitals, so the numbers do not apply to everyday infections. But they show why careful antibiotic use matters.

Expert input can make a real difference. For serious bloodstream infections caused by the germ Staphylococcus aureus, studies found about 12 deaths per 100 patients when an ID specialist was involved, compared with about 26 per 100 without.¹³ These studies cannot fully prove cause and effect, but the difference is large.

Behind the scenes: protecting every patient in the hospital

Much of an ID specialist’s work happens away from the bedside, in roles that protect everyone in a hospital.

Antimicrobial stewardship

This means making sure antibiotics are used only when needed, and then at the right dose and for the right length of time. ID specialists help write hospital antibiotic rules, review prescriptions, and advise when a medicine can be stopped or switched to a simpler one.

It works. A major review of 221 studies found that such programmes shorten antibiotic courses, from about 11 to 9 days per patient, without any increase in deaths, and probably shorten hospital stays slightly.¹⁴ Fewer unnecessary antibiotics mean fewer side effects and less resistance.

Infection prevention and control

ID specialists also work with infection control teams on hand washing, the safe use of drips and catheters, isolating patients with resistant or contagious infections, and investigating outbreaks.

A review of studies from low- and middle-income countries found that about 15 out of every 100 hospital patients pick up a new infection during their stay.¹⁵ Another review of 144 studies found that infection-control programmes, such as better hand hygiene and safer use of catheters and drips, cut these infections by 35% to 55%.¹⁶

The two roles work together: fewer hospital infections mean fewer antibiotics are needed, and wiser antibiotic use means fewer resistant germs. The ICMR study above called for both to be strengthened urgently in India.¹²

When should you see an Infectious Diseases specialist?

Ask your doctor whether it would help if:

you have had a fever for more than three weeks and tests have not found the cause
an infection is not improving with treatment, or keeps coming back
your report shows a resistant germ, with words like MRSA, ESBL or carbapenem-resistant
you have a serious infection of the blood, heart valves, brain, bones, joints or an implant
you have HIV, TB, or hepatitis B or C
your immunity is weak, or you are about to start a transplant, chemotherapy or a biologic medicine
you fell ill after travel, or have a tropical fever that is hard to pin down
you need adult or travel vaccines, or preventive treatment after an exposure such as a needle-stick injury

You don’t need a rare or complicated infection to see an ID specialist. Most ID doctors are just as happy to look after common, everyday infections, because caring for infections of every kind is what they enjoy about their work.

Getting the most out of your visit

Bring all your reports: blood tests, culture reports, scans and discharge summaries.
Bring a list, or photos, of every medicine you have taken for this illness, with dates.
Tell the doctor about all your other medicines, including herbal ones.
Mention recent travel, contact with animals, hospital stays, and any implants.
Don’t start antibiotics on your own before tests are done.

In India, most people see an ID specialist through a referral from their doctor, usually in larger hospitals and medical colleges. Free tests and treatment for HIV and TB are also available through government programmes.

Don’t wait: get emergency care for these warning signs

An infection can sometimes turn into sepsis, a life-threatening emergency. Call 112 or go to the nearest emergency department straight away if someone with an infection has:

confusion, slurred speech or unusual drowsiness
severe breathlessness or very fast breathing
dizziness or feeling faint
cold, clammy, pale or mottled skin
passed no urine for a day
got worse quickly, or feels very unwell

Every hour counts. The Surviving Sepsis Campaign’s 2026 international guidelines treat sepsis as a medical emergency, and recommend that people with probable sepsis or septic shock receive antibiotics immediately, ideally within one hour of it being recognised.¹⁷

Key points

Most infections are treated well by your regular doctor.
An Infectious Diseases specialist helps when infections are serious, unusual, unclear or resistant to treatment.
Lab results need careful reading: a negative test does not always rule out an infection, and a positive test does not always mean an active one.
Infection medicines can interact with other medicines, so always share your full medicine list.
People with weak immunity need extra care, because their infections can be unusual, quiet and fast-moving.
Behind the scenes, ID specialists protect every hospital patient through stewardship and infection control.
Warning signs of sepsis need emergency care, not an appointment.

References

This article is for general education and is not a substitute for advice from a doctor who knows your medical history. Please read the medical disclaimer.

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