kansen ha sinkoku desu.

It is read:

かんせん は しんこく です
Romanization: kansen wa shinkoku desu

A useful detail: the particle は is written ha but pronounced wa when it marks the topic.

は is the topic marker. It tells you what the sentence is about.

So 感染は means something like:

• as for infection
• regarding infection
• infection, ...

The sentence structure is basically:

[topic] は [comment] です

So here:

• 感染 = infection
• は = topic marker
• 深刻です = is serious

When は is used as a particle, it is pronounced わ / wa. This is just a standard feature of modern Japanese spelling and pronunciation.

So:

• はな = hana if it is part of a word
• 感染は = kansen wa when は is the topic particle

This also happens with a couple of other particles in Japanese, such as へ often being pronounced e when used as a particle.

深刻 is a na-adjective (also called an adjectival noun).

That means it behaves differently from an i-adjective.

Examples:

• 深刻な問題 = a serious problem
• 問題は深刻です = the problem is serious

So in your sentence, 深刻 is the predicate, followed by です.

Because な is only used when a na-adjective directly modifies a noun.

Compare:

• 深刻な感染 = a serious infection

  • here 深刻 modifies the noun 感染, so you need な

• 感染は深刻です = the infection is serious

  • here 深刻 is the main predicate, so you do not use な

So:

• before a noun: 深刻な + noun
• at the end of a sentence: 深刻です

Because 深刻です is the predicate of the sentence.

Japanese often ends polite sentences with:

• a noun + です
• a na-adjective + です
• an i-adjective by itself (or sometimes with polite forms)

Here, 深刻です means is serious. No extra particle is needed between 深刻 and です.

It is similar, but not exactly identical.

In this sentence, です works like a polite copula, so translating it as is is fine:

• 感染は深刻です = The infection is serious / Infection is serious

But です is really more of a politeness-and-predication element than a perfect one-to-one match for English is.

Also, this form is polite and non-past.

Yes, but the nuance changes.

• 感染は深刻です
  = As for infection, it is serious
  This sounds like a general statement about the topic.

• 感染が深刻です
  = The infection is what is serious / Infection is serious with more focus on infection as the subject
  This can sound more contrastive or more like answering a question such as What is serious?

For a neutral statement in news, reports, or general explanation, は is very natural.

It most basically means infection, but the exact English wording depends on context.

In real Japanese, 感染 can refer to:

• infection as a general concept
• becoming infected
• the state or issue of infection
• in some contexts, infection spread

So depending on the situation, English might render it as:

• infection
• infections
• the outbreak
• the spread of infection

Japanese often leaves that level of specificity to context.

Japanese often does not state subjects the way English does.

In this sentence, 感染 is the topic, and the sentence comments on it. There is no need to add a word like it.

English often requires a more explicit subject, but Japanese does not. So a natural Japanese sentence can be much more compact.

The plain form is:

感染は深刻だ。

In conversation, people may also say:

感染は深刻。

That sounds more casual or clipped.

So the levels are roughly:

• 感染は深刻です。 = polite
• 感染は深刻だ。 = plain
• 感染は深刻。 = casual / abbreviated style

The pattern is:

Noun は na-adjective です

In this sentence:

• 感染 = noun
• は = topic marker
• 深刻 = na-adjective
• です = polite sentence ending / copula

So it follows a very common beginner pattern:

X は Y です
where Y can be a noun or a na-adjective.

That makes this a useful model sentence for many similar expressions, such as:

• 問題は重要です。 = The problem is important.
• 状況は複雑です。 = The situation is complicated.